We have confirmed and extended our earlier findings on the evolutionary relationship between different oncovirus genera and oncoviral related sequences in human cellular DNA. By the combined criteria of low stringency blot hybridization and comparative nucleotide sequence analysis, we have established the existence of two major pol gene families in the evolution of oncoviruses. One family is composed of mammalian type C viruses. The other family includes type A, B, D, avian type C and human T-cell leukemia virus (HTLV). The major region of homology between members of the latter family could be localized to the 3' end of the pol gene. In the avian type C, Rous Sarcoma virus, this region corresponds to a pp30 peptide which has endonucleolytic activity that is highly specific for closed circular proviral DNA. Nucleic acid sequence homology could also be demonstrated between the env genes of mammalian type C and type D oncoviruses. This data provides evidence for the genetic interaction between the progenitors of mammalian type C and type D oncovirus genera and suggests that this phenomenon may be an active force in the evolution of oncoviruses. In previous work we have described recombinant clones of human cellular DNA which contained MMTV related sequences. The major region of homology corresponds to the pol gene of MMTV. The corresponding nucleotide sequence of one human recombinant clone (designated HLM-2) has been determined. Within a 524 base pair segment HLM-2 shares 51, 50, 44, and 37 percent homology with respectively MMTV (type B), SMRV (type D), RSV (avian type C) and HTLV oncoviruses. There was no significant nucleotide sequence homology with the Moloney leukemia virus pol gene. These results confirm our earlier findings using low stringency blot hybridization.